The present invention relates generally to the testing of integrated circuit dies on a wafer during manufacture. More specifically, but without limitation thereto, the present invention relates to reducing the variation of a selected parameter in a production lot of integrated circuit die.
An important issue in the manufacture of integrated circuits is detecting and rejecting integrated circuit die replicated on a silicon wafer that exhibit values of a selected parameter, for example, quiescent current (Iddq), that differ significantly from a mean value of the parameter. Integrated circuit die having values of the selected parameter that differ from the mean value by more than a selected threshold are called statistical outliers. Statistical outliers may pass performance testing, however, they may be more subject to premature failure and thus reduce the average service life of a production lot. Accordingly, a need exists for a method of detecting the statistical outliers.
In one aspect of the present invention, a method of detecting spatially correlated variations includes measuring a selected parameter of each of a plurality of electronic circuits replicated on a common substrate; calculating a difference between a value of the selected parameter at a target location and that of an identical relative location with respect to the target location for each of the plurality of electronic circuits to generate a distribution of differences; calculating an absolute value of the distribution of differences; and calculating an average of the absolute value of the distribution of differences to generate a residual for the identical relative location.
In another aspect of the present invention, a process for reducing the variation of a selected parameter of an integrated circuit die includes measuring a selected parameter of each of a plurality of integrated circuit die replicated on a wafer substrate; calculating a difference between a value of the selected parameter at a target location and that of an identical relative location with respect to the target location for each of the plurality of integrated circuit die to generate a distribution of differences; calculating an absolute value of the distribution of differences; calculating an average of the absolute value of the distribution of differences to generate a residual for the identical relative location that is representative of an expected value range of the selected parameter at the identical relative location; and rejecting any of the plurality of integrated circuit die having a value of the selected parameter that lies outside the expected value range.